There have been used constant-voltage power supply units for providing a predetermined constant voltage by controlling a dc input voltage by means of a primary control transistor. Such constant-voltage power supply unit has an error amplifier adapted to obtain the difference between the output voltage and a reference voltage, wherein the primary control transistor is controlled on the basis of the difference such that the output voltage remains at the predetermined constant voltage. The voltage supply unit may have an over-current protection function for suppressing below a predetermined level an over-current caused by, for example, malfunctions of a load. Japanese Patent Early Publication No.2002-304225 discloses an over-current protection function characterized by not only a current drooping characteristic but also a so-called fold-back characteristic for reducing the output current in the event the output voltage has dropped.
Since a constant-voltage power supply unit has a fold-back type over-current protection function adapted to provide a predetermined constant voltage when the output current is within allowable limits and reduce the output current along with the output voltage (over-current protection mode) when the output current has reached a maximum allowed level, the unit can advantageously minimize energy loss while operating in the over-current protection mode.
It is necessary for the fold-back type over-current protection function to determine a proper protective current level independently of ambient temperature and use conditions, set a minimum allowable current level in the over-current protection mode, and provide a predetermined offset to secure a normal startup of the power supply unit as needed.
In conventional constant-voltage power supply units, the offset level is determined based on the potential drop across a resistor or a diode, which is, however, greatly influenced by ambient temperature and use condition. As a consequence, it is difficult to properly determine and set a protective current level. Moreover, extra power consumption is inevitable during the over-current protection mode, since the permissible current level in the over-current protection mode must allow for an extra margin.
In recent years, a ceramic capacitor has been increasingly used as a smoothing capacitor connected on the load side of the output terminal of the power supply unit, because a ceramic capacitor has not only good reliability and durability but also a larger capacity per unit volume than other capacitor such as a tantalum capacitor and an electrolytic capacitor, which enable production of a miniaturized yet lugged capacitor. As a consequence, following a recent trend of miniaturization and energy saving policy on electric devices, most of capacitors used in the electric devices are ceramic capacitors such as lamination type capacitors. However, ceramic capacitor has a disadvantage that its equivalent series resistance (ESR) is remarkably small as compared with that of a tantalum capacitor and an electrolytic capacitor.
From an energy saving point of view, it is preferable for the capacitor to have a small ESR since small ESR implies small energy consumption. However, in performing high-speed voltage feedback of a constant-voltage power supply unit, it is difficult to acquire a sufficiently large feedback signal for ac components if the ESR is small, though necessary for phase compensation. Moreover, if the amplification of the relevant feedback loop is stepped up to amplify the feedback signal, a new problem arises in that the control loop becomes more likely to suffer oscillations.